The present invention relates generally to digital transmission and reception systems and particularly to a digital data transmission and reception system having a data frame structure and circuit arrangement selected to facilitate operations such as symbol to byte and byte to symbol conversion, interleaving and deinterleaving, and forward error correction. The system also facilitates the use of a data rate that is related to the signal to noise ratio (S/N ratio) of the transmission environment for enhancing system capacity.
The present invention also relates to the use of trellis coded modulation (TCM) in transmission and reception systems and particularly concerns the use of TCM in high definition television (HDTV) applications.
U.S. Pat. Nos. 5,087,975 and 5,600,677 disclose a vestigial sideband (VSB) system for transmitting a television signal in the form of successive M-level symbols over a standard 6 MHZ television channel. The television signal may, for example, comprise one or two compressed wideband HDTV signals or a number of compressed lower resolution signals. While the number of levels, M, characterizing the symbols may vary depending on circumstances, the symbol rate is preferably fixed, such as at 10.76 Megasymbols/sec. The number of symbol levels used in any particular situation is largely a function of the S/N ratio characterizing the transmission medium. For example, where the S/N ratio is low, a smaller number of symbol levels may be used. It is believed that the ability to accommodate symbol levels of 16, 8, 8 with trellis coding (8 VSBT), 4, and 2 provides adequate flexibility to satisfy conditions in most systems. It will be appreciated that lower values of M can provide improved S/N ratio performance at the expense of reduced transmission bit rate. For example, assuming a rate of 10.76 Megasymbol/sec, a 2-level VSB signal. (1 bit per symbol) provides a transmission bit rate of 10.76 Megabits/sec, a 4-level VSB signal (2 bits per symbol) provides a transmission bit rate of 21.52 Megabits/sec, and so on up to a 16-level VSB signal which provides a transmission bit rate of about 43.04 Megabits/sec.
It is generally known that the S/N ratio performance of cable television plants decreases as the signal (channel) frequency increases. The foregoing attribute of an M-level VSB transmission system, i.e., improved S/N ratio performance as M decreases, is used in one aspect of the invention to compensate for the S/N ratio degradation in the higher frequency channels of CATV distribution plants. That is, according to this aspect of the invention, VSB transmission is effected in a CATV system wherein the lower frequency channels are transmitted using larger values of M. While the bit rate of the higher frequency channels is thereby reduced, the received signal may be reproduced with a S/N ratio comparable to that of the lower frequency channels.
It is also generally known that the S/N performance of digital signals broadcast over the air may be improved by TCM (trellis coded modulation). U.S. Pat. Nos. 5,600,677 and 5,583,889 describe an 8 level TCM coded VSB signal. A Viterbi decoder in the receiver is used in close cooperation with a comb filter (disclosed in U.S. Pat. No. 5,087,975). The comb filter rejects co-channel interference caused by existing NTSC signals.
Moreover, in accordance with other aspects of the invention, system efficiency, particularly in relation to such operations as data interleaving and deinterleaving, symbol to byte and byte to symbol conversion, forward error correction, and Viterbi decoding, may be greatly enhanced by selecting a data frame structure which facilitates these operations within the constraints of the variable M-level VSB character and TCM coding parameters of the transmitted signal. U.S. Pat. No. 5,677,911 discloses a data frame structure for a 6 MHZ channel.
This application and the other copending applications described above adapt the previously disclosed VSB system so that it can be transmitted over standard 8 MHZ television channels (as used in China and Europe) with the ability to reject interference caused by existing PAL signals. In this system the symbol rate is preferably 14.14 Megasymbols/sec so that all bit rates increase proportionately.
Trellis coded modulation is a well known technique for improving the performance of digital transmission and reception systems. For example, improvements can be achieved in signal to noise (S/N) performance at a given power level; alternatively, the transmitted power required to achieve a given S/N performance can be reduced. In essence, TCM comprises the use of a multi-state convolutional encoder to convert each k input data bits of an input sequence of data bits into k+n output bits, and is therefore referred to as a rate k/(k+n) convolutional encoder. The output bits from the convolutional encoder are then mapped into discrete symbols (having 2(k+n) values) of a modulated carrier for data transmission. The symbols may, for example, comprise 2(k+n) phase or amplitude values. By encoding the input data bits in a state-dependent sequential manner, increased minimum Euclidean distances between the allowable transmitted sequences may be achieved leading to a reduced error probability when a maximum likelihood decoder (e.g., a Viterbi decoder) is used in the receiver.
FIG. 1 generally illustrates a system of the type described above. Each k bits of an input data stream is converted to k+n output bits by a rate k/(k+n) state-dependent sequential convolutional encoder 10. Each group of (k+n) output bits is then mapped by a mapper 12 to a symbol having a corresponding one of 2(k+n) levels. The symbols are transmitted over a selected channel by a transmitter 14. A receiver includes a tuner 16 for converting the signal received over the selected channel to an intermediate frequency signal, which is demodulated by a demodulator 18 to provide a baseband analog signal. The analog signal is appropriately sampled by an analog to digital converter (A/D) 20 in order to recover the transmitted symbols which are then applied to a Viterbi decoder 22 for recovering the original k data bits.
U.S. Pat. No. 5,087,975 also discloses the use of a receiver comb filter having a subtracting element and a feed forward delay of twelve symbol clock intervals for reducing NTSC co-channel interference in the receiver. In order to facilitate operation of the receiver comb filter, the source data is precoded by a modulo-filter having a feedback delay of twelve symbol clock intervals. (In the absence of significant NTSC co-channel interference, the receiver of the patented system may include a complementary modulo postcoder which is used to process the received signal in lieu of the comb filter in order to avoid the degradation of S/N performance attributable thereto.) A system using TCM and the above comb filter is disclosed in the ATSC digital television standard published on Sep. 16, 1995 and in U.S. Pat. Nos. 5,600,677 and 5,583,889.
In a system using TCM and a comb filter, each pair of input data bits is supplied to a precoder and trellis encoder. One of the bits in each pair of bits is supplied to the precoder, and the other of the bits in each pair of bits is supplied to the trellis encoder. The precoder and trellis encoder each incorporates one or more twelve bit delay elements. Thus, the precoder and trellis encoder may be envisioned as twelve identical precoders and trellis encoders with (i) an input commutator (i.e., demultiplexer) for sequentially connecting input sets of two bits to the twelve identical precoders and trellis encoders and (ii) an output commutator (i.e., multiplexer) for sequentially connecting output sets of three bits to a symbol mapper.
The twelve precoders and trellis encoders interleave the bit pairs so that each bit pair in a first byte of data is processed by a first precodet and trellis encoder, so that each bit pair in a second byte of data is processed by a second precoder and trellis encoder, . . . and so that each bit pair in a twelfth byte of data is processed by a twelfth precoder and trellis encoder. Each subsequent sets of twelve bytes are similarly processed. The symbol mapper maps each set of three output bits to a symbol having a corresponding one of eight signal levels of an eight-level constellation. The resulting symbols are supplied to a multiplexer which adds synchronization symbols to the data symbols in order to structure the data and synchronization symbols in a frame.
A frame for a 6 MHZ channel is structured so that it has 313 segments. The first segment of a frame (a frame sync segment) includes (i) a segment sync portion containing four segment sync symbols and (ii) a field sync portion containing 828 pseudo-randomly generated field sync symbols. Each of the other 312 segments (data segments) includes (i) a segment sync portion containing four segment sync symbols and (ii) a data portion containing 828 symbols of data.
Thereafter, the symbols in the above described frame structure are transmitted, and are received by a receiver. The receiver includes the comb filter and a trellis decoder. The comb filter is present in order to filter out interference which may be caused by NTSC channels broadcast by nearby stations. The trellis decoder (such as a Viterbi decoder) is present in order to decode the symbols in the received frames into their corresponding original bit pairs. The trellis decoder is similar to the trellis encoder in that the trellis decoder processes the symbols of the same byte together. Thus, these symbols must enter the trellis decoder in the correct sequence.
The present application and the other copending applications mentioned above relate to a modification of the above 6 MHZ VSB system so that it will operate over standard 8 MHZ television channels and have the ability to reduce PAL co-channel interference. In the system of the present application, the symbol rate is preferably fixed at about 14.14 MHZ (instead of 10.76 MHZ). Also a nine way (instead of a twelve way) trellis encoding process is utilized, and the data frame consists of 289 segments (instead of 313 segments). Both the prior disclosed VSB system and the VSB system disclosed in the present application utilize multiple modes, which are described in Part I below, and in U.S. Pat. No. 5,677,911.